The present invention relates to a method of synthesizing nickel fibers, particularly to a chemical method of synthesizing nickel fibers by oxidation-reduction.
In the past few years, nickel has been widely used in the industries. Its major applications include the preparations of mechanical alloy materials, ceramic materials, magnetic materials and catalysts. In the electrical and electronic products, nickel has been used in the preparation of conductive adhesives, EMI shielding and anti-electrostatic materials. Its applications in both the conductive adhesives and EMI shielding materials, nickel has been used as a conductive filler. When the concentration of such a conductive filler in a matrix reaches a threshold, the resistance thereof will have an abrupt decrease thereby changing the matrix from an insulator to a conductor. Such a threshold is called a percolation threshold. However, this threshold will increase when this type of conductive filler is blended with a resin or a plastic, due to poor blending effects. Therefore, in an ordinary process for preparing the conductive adhesives or EMI shielding products, the amount of conductive filler used in the form of particles usually reaches more than 70% by volume in order to assure the electric quality. However, an increase in the amount of conductive filler used means an increase in the cost.
According to reports in the literature, the methods for reducing the percolation threshold of conductive filler mainly comprise: improving the mixing between the conductive filler and the matrix, using a powder form conductive filler with a larger surface area, i.e. a finer powder, and using a conductive filler with a higher aspect ratio, e.g. in the form of flake, fiber and filament.
Currently the methods of synthesizing metal powders include the physical synthesis method and the chemical synthesis method. Common physical synthesis methods include pulverizing and sputtering, etc.; while common chemical synthesis methods include sol-gel, vapor deposition, and the oxidation-reduction method. Take nickel for an example, in the literature or patents, there is no disclosure related to chemical method of synthesizing nickel fibers, and there are only methods of synthesizing nickel powder. In the methods of synthesizing nickel powder, the first approach comprise reducing a nickel precursor with a gaseous reducing agent in a high temperature, e.g. U.S. Pat. Nos. 3,850,612; 4,673,430; and 5,584,908.
The xcex3-ray irradiation reduction method uses Co60 as the radiation source. After irradiating a NiSO4 aqueous solution, nickel ions can be reduced into metal nickel particles. In this method an OH free radical scavenger is added to avoid the monovalent nickel ions from being oxidized into bivalent ions in the solution phase reactions. Otherwise, the yield will be reduced. Furthermore, an increase in the pH value of the reaction mixture will increase the yield. The literature disclosure includes:
Marignier et. al. [Marignier, J. L., J. Belloni, M. O. Delcourt and J. P. Chevalier, Microaggregates of Non-noble Metals and Bimetallic Alloys Prepared by Radiation-induced Reduction, Nature, 317, 344 (1985)] use NiSO4 as the precursor, iso-propanol as the scavenger, and PVA as the surfactant. After xcex3-ray irradiation, the product has a particle size of 10-20 nm, and the Ni particles have a face-centered cubic (fcc) crystal structure.
Zhu et al. [Zhu, Y. J., Qian Y., Zhang M. W., Chen Z. Y., Chen M. and Zhou G., Preparation and Characterization of Nanocrystalline Nickel Powders by the xcex3 Radiation Method, J. Mater. Sci. Lett., 13, 1243 (1994)] add SDS into the reaction solution as the surfactant, and further add NH4OH as the alkaline source to maintain the pH of the solution at 10-11 during the reaction thereby inhibiting the oxidation reaction. The yield can reach 90%. The Ni product has a fcc structure with an average particle size of 8-9 nm.
The oxidation-reduction methods can be divided mainly into two classes based on the solvent used, one is deionized water and the other is alcohol. A synthesis method using an alcohol as the solvent is also called a polyol process, in which the solvent is also used as a reduction agent. The polyol methods of synthesizing nickel particles include:
Hegde et al. [Hedge, M. S., D. Larcher, L. Dupont, B. Beaudoin, K. Tekaia-Elhsissen and J. M. Tarascon, Synthesis and Chemical Reactivity of Polyol Prepared Monodispersed Nickel Powders, Solid State Ionics, 93, 33 (1997)] use Pd or Pt as a nucleation agent, PVP as a protection agent, and ethylene glycol as a solvent, in which nickel hydroxide is reduced at 198xc2x0 C. for 30-40 minutes to yield nickel particles 135 nm in size.
Kurhara et al. [Kurihara, L. K., G. M. Chow and P. E. Schoen, Nanocrystalline Metallic Powders and Films Produced by the Polyol Method, Nanostructured Mater., 5(6), 607 (1995)] disclose that Ni crystals about 20 nm in size can be obtained when a concentration of nickel nitrate of 0.02-0.2M, a reaction temperature of 120xc2x0 C. and a reaction time of one hour are used.
Typical aqueous phase reactions are:
U.S. Pat. No. 4,089,676 disclose a method of synthesizing micro nickel particles, wherein a nickel precursor is reduced by hydrazine at 88-96xc2x0 C. in the presence of an alkaline and an antifoam agent made by the Union Carbide Company.
U.S. Pat. No. 3,923,496 uses oxalic acid to reduce nickel nitrate in a nitrogen or carbon dioxide environment thereby obtaining nickel particles over 0.5 xcexcm in size. If an inhibitor (e.g. magnesium oxide) is added, the particle size can be reduced to 0.1 xcexcm.
The methods currently used by the industry for synthesizing nickel fibers include:
(a) Metal nickel, after being molten, is drawn directly to form a filament. However, this method requires a high temperature for heating and the nickel filament can not reach a very small diameter.
(b) Shui and Chung [Shui, X. P. and D. D. L. Chung, Submicron Nickel Filaments Made by Electroplating Carbon Filaments as a New Filler Material for Electromagnetic Interference Shielding, J. Electronic Mater., 24 (2), 107 (1995)] electroplates nickel metal on a carbon fiber, wherein the synthesized nickel fibers can reach a diameter of 0.4 xcexcm and a length of 100 xcexcm when the volume fraction of nickel is 94%. However, this method uses a carbon fiber; therefore, the method is restricted by the dimensions of the carbon fiber. Furthermore, since the electroplating technique is used, the product is difficult to be uniform as a result of the distribution of the electric field. Moreover, the nickel filament produced according to said article still has a carbon core which has a poorer conductivity than a pure nickel filament. The nickel filament synthesized according to said article mainly is used for its EMI effects. Theoretically, better results will be accomplished if the diameter of the metal filament can be further reduced.
Ichiki et al. [Ichiki, M., J. Akedo, K. Mori and Y. Ishikawa, Microstructure of Nickel Whiskers Produced by the Gas Deposition Method, J. Mater. Sci. Lett., 16, 531 (1997)] use a vapor phase deposition method to grown nickel whisker, which comprises evaporating nickel particles by heat (1700xc2x0 C.), cooling the vapor by an inert gas (e.g. Ar or He), and depositing nickel on a substrate through a nozzle. The nickel filament produced according to said vapor phase deposition technique has a diameter of 100 xcexcm and a length of 3 mm. However, said vapor phase deposition technique requires a high capital investment in the facilities. Furthermore, the yield will not be too high due to a low concentration in the vapor phase.
The main objective of the present invention is to provide a chemical method of synthesizing nickel fibers, particularly an oxidation-reduction method in a solution phase.
The present invention basically uses hydrazine as a reducing agent, a solution of nickel salt as a precursor, and suitable additives, to carry out a reduction reaction of nickel ions below 100xc2x0 C., wherein the reduced nickel is controlled in the form of a filament instead of a powder by using a magnetic field or a surfactant. The nickel fibers produced accordingly can have a length of up to several centimeters; and the reaction will not be too fast to an extent that a portion of the reduced nickel metal deposits on the wall of the reactor causing a reduction in yield. The nickel fibers synthesized according to the present invention can be recovered from the reaction mixture through simple separation and drying; and the nickel fibers so obtained can have a diameter of microns to sub-microns and a length of centimeters.
The synthesis method proposed by the present invention is a liquid phase oxidation-reduction method and has a lower cost in facilities and a faster reaction rate in comparison with the prior art methods of producing nickel filaments. Furthermore, the nickel fibers produced are very easy to be separated from the reaction solution; therefore, the present method is suitable for commercial mass production.